This invention relates to a feedback control method of controlling the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine, and more particularly to a method of this kind, which is applied when it is detected that the engine has entered a feedback control effecting region.
A fuel supply control method for an internal combustion engine, particularly a gasoline engine, has been proposed, e.g. by U.S. Pat. No. 4,445,482 issued May 1, 1984, which is adapted to determine the valve opening period of a fuel injectin device for control of the fuel injection quantity, i.e. the air-fuel ratio of an air-fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine speed and intake pipe absolute pressure and then adding to and/or multiplying same by variables and/or coefficients indicative of operating conditions of the engine, such as engine speed, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentation), etc., by electronic computing means.
According to this proposed method, while the engine is operating in a normal operating condition, the air-fuel ratio is controlled in closed loop or feedback mode such that the valve opening period of the fuel injection device is controlled by varying the value of a coefficient in response to the output of an exhaust gas ingredient concentration detecting means which is arranged in the exhaust system of the engine, so as to attain a theoretical air/fuel ratio or a value close thereto (closed loop control), whereas while the engine is operating in one of particular operating conditions (e.g. a mixture-leaning region, a wide-open-throttle region, and a fuel-cut effecting region), the air-fuel ratio is controlled in open loop mode by the use of a mean value of values of the above coefficient applied during the preceding feedback control, together with an exclusive coefficient corresponding to the kind of the particular operating region in which the engine is then operating, thereby preventing deviation of the air-fuel ratio from a desired air-fuel ratio due to variations in the performance of various engine operating condition sensors and a system for controlling or driving the fuel injection device, etc., which are caused by machining tolerances or the like and/or due to aging changes in the performance of the sensors and the system, and also achieving required air-fuel ratios best suited for the respective particular operating conditions, to thus reduce the fuel consumption as well as improve the driveability of the engine.
However, according to this method, the mean value of values of the above coefficient which have been applied during the preceding feedback control assumes a different value each time it is calculated and stored at each different operating point of the engine within the region wherein feedback control should be effected. As a result, in the case that the feedback control effecting region is previously divided into a plurality of subdivided regions, when the engine operating point shifts from one of the subdivided regions to another one, there exists a time lag, i.e. a feedback control lag before the above feedback control correction coefficient assumes a value appropriate for attaining desired emission characteristics for the another subdivided region during the feedback control. Therefore, if the method is applied to an internal combustion engine having an exhaust gas purifying device, such as a three-way catalyst, until a period of time corresponding to the above time lag elapses, the amount of an exhaust gas ingredient NOx can increase if the air-fuel ratio varies from a leaner value to an appropriate value for attaining the desired emission characteristics, whereas the amounts of ingredients CO, UHC, etc. in the exhaust gases can increase if the air-fuel ratio varies from a richer value to the same appropriate value.